WASHINGTON -- Former White House national security adviser John M. Poindexter, a central figure in the Iran-contra affair, appeared to make significant headway in federal court yesterday with a broad challenge to his conviction for five crimes during the scandal.

In a hearing that ran more than an hour longer than scheduled, two of the judges on the three-member panel of the U.S. Circuit Court of Appeals here repeatedly asked questions sympathetic to the retired Navy rear admiral's legal claims.

One of the two, Circuit Judge David B. Sentelle, at times appeared to be taking on the role of direct adversary to the lawyer for special prosecutor Lawrence E. Walsh, explicitly undercutting the prosecution's points -- often with sarcasm, sometimes with outright ridicule.

Mr. Walsh's lawyer, Andrew L. Frey, was pressed so aggressively at times that he began interrupting Judge Sentelle in midquestion.

A key facet of all three of the judges' questions was the role played in Poindexter's trial by the prosecution's star -- but reluctant -- witness, Marine Lt. Col. Oliver L. North, who was Poindexter's deputy on President Ronald Reagan's White House staff.

Circuit Judge Douglas H. Ginsburg at one point wondered if the entire conviction of Poindexter ought to be thrown out if the Circuit Court found that the role Mr. North played was legally "improper" -- a conclusion his and Judge Sentelle's questions indicated was a likelihood. Mr. Frey struggled to find a way to suggest that the three-judge panel would not have to go that far.

The questioning yesterday was dominated by Judge Sentelle. He was a member of another appeals court panel last year that set aside Mr. North's conviction for his key role in the Iran-contra scandal. The special prosecutor has now taken the North case on to the Supreme Court, seeking to reinstate the North conviction.

Of those who have been convicted or pleaded guilty to criminal charges growing out of the scandal, only Poindexter has received a prison sentence -- six months. He is free pending the outcome of his appeal.

The key legal issue that worked in Mr. North's favor in the appeals court -- a claim that testimony he was forced to give to Congress under a grant of immunity was used against him illegally -- is also being used by Poindexter, who similarly was promised immunity for congressional testimony in 1987.

Mr. Frey tried several times yesterday to say that the Poindexter case was different, and that the trial judge in his case had done more than enough to assure that the admiral's Capitol Hill testimony was not used in ways that would "taint" that trial. But Judge Sentelle, and at times Judge Ginsburg, reacted skeptically to that claim.

The third member of the new panel, Chief Judge Abner J. Mikva, occasionally joined in the display of skepticism about North as a witness but largely left the pummeling of Mr. Frey to his colleagues.

The Circuit Court is expected to study the case for several weeks before issuing a ruling.